The so-called axial transmission technique has been used to assess long bones for over four decades (see, for example, Gerlanc et al, Clin. Orthop. Rel. Res. 1975; 111:175-180). With this method, an ultrasonic pulse is transmitted along the long axis of a bone (typically the tibia) from a transmitter to a receiver and the velocity is estimated from the transit time of the first arriving signal and the propagation distance. To account for the effects of overlying soft tissue, either a multiple transmitter/receiver configuration can be used , or transit time can be determined as a function of distance as one transducer is moved relative to the other . At least two commercial clinical devices for bone assessment using axial ultrasound transmission have been produced: the Soundscan 2000/Compact (Myriad Ultrasound Systems Ltd., Rehovot, Israel) operating at 250 kHz, and the Omnisense (Sunlight Medical Corp., Rehovot, Israel) operating at 1.25 MHz (see patent no. WO 99/45348). A recent investigation by Camus et al (J Acoust. Soc. Am. 2000; 108:3058-3065) into the axial transmission technique indicated that, under certain conditions, the first arriving signal can correspond to a lateral wave (or head wave) propagating along the surface of the solid at the bulk longitudinal velocity. The conditions under which lateral waves were observed included an appropriate measurement geometry (in terms of the separation of the transducers and their distance from the surface), an approximately point-like transmitter and receiver (spherical wavefronts), and the use of wavelengths less than the thickness of the solid layer. Tibial ultrasound velocity values measured in vivo are comparable to, or slightly lower than, in vitro measurements of the axial longitudinal wave velocity in excised human cortical bone specimens. However, there is experimental evidence indicating that the velocity of the first arriving signal is lower than the longitudinal velocity when the wavelength is greater than the bone thickness. Simulation studies show similar trends, and indicate that the waves contributing to the first arriving signal change as the sample becomes thinner. Clinical evidence for such thickness effects is, as yet, inconclusive, but this may be due to differences in the ultrasonic frequencies used by the different commercial systems, or other methodological factors.
Tibial ultrasound velocity measured using current commercial devices correlates with tibial bone mineral density (BMD), and, to a lesser extent with BMD at other skeletal sites (see, for example, Foldes et al. Bone 1995; 17:363-367), and also reflects cortical bone elastic modulus. However, tibial ultrasound is a poor discriminant of osteoporotic fracture, and is only weakly correlated with femoral strength and BMD. There are a number of reasons why current tibial ultrasound measurements may be sub-optimal in terms of their sensitivity to relevant bone properties. Waves propagating at the bone surface may preferentially reflect the material properties of bone in the periosteal region. In osteoporosis, cortical bone changes occur primarily in the endosteal region. The porosity of endosteal bone increases leading eventually to endosteal resorption, “trabecularisation”, and thinning of the cortex. In addition, recent nanoindentation studies suggest there may be differential changes with aging in the elastic properties of periosteal and endosteal bone purely at the material level. Ultrasonic methods that target these known pathological changes are likely to prove more valuable clinically. A further concern is that if density and elasticity both exhibit a parallel change, for example as a result of a change in porosity, ultrasound velocity may not be altered because the two effects tend to cancel out (since longitudinal velocity varies as the square root of elasticity divided by density). These considerations suggest that any improved ultrasonic method for cortical bone assessment should be sensitive to one of more of the following factors: a) reduced cortical thickness, b) structural changes in the endosteal region, such as increased porosity, and c) changes in bone density and elasticity at the material level, ideally independently of each other.
In general, little consideration has been given to the possibility of using different types of ultrasonic waves in long bones. One exception has been work reporting low frequency ultrasonic measurements of “surface wave” velocity in the tibia, mapping the spatial variation in velocity (Jansons et al. Biomaterials 1984; 5:221-226). However, since pure surface waves only exist in structures that are much thicker than the wavelength, it is likely that these researchers were actually measuring a guided wave mode reflecting both bone thickness and material properties. Guided waves propagate within bounded or layered media, and their characteristics are determined by the geometrical and material properties of the structure and of the surrounding media They arise from the reflection, mode conversion and interference of longitudinal and shear waves within the structure (Victorov L. A. Rayleigh and Lamb Waves. New York, Plenum, 1967). Ultrasonic guided waves are quite widely used in engineering non-destructive testing for the assessment of plates, tubes and more complex structures.